Data is always written in ‘C’ order, independent of the order of a.
The data produced by this method can be recovered using the function
fromfile().

Parameters:

fid : file or str

An open file object, or a string containing a filename.

sep : str

Separator between array items for text output.
If “” (empty), a binary file is written, equivalent to
file.write(a.tostring()).

format : str

Format string for text file output.
Each entry in the array is formatted to text by first converting
it to the closest Python type, and then using “format” % item.

Notes

This is a convenience function for quick storage of array data.
Information on endianness and precision is lost, so this method is not a
good choice for files intended to archive data or transport data between
machines with different endianness. Some of these problems can be overcome
by outputting the data as text files, at the expense of speed and file
size.